(1) Field of the Invention
The present invention relates to apparatuses which perform scanning using laser beams, and particularly to a display apparatus such as a head-up display (HUD) and a head-mounted display (HMD).
(2) Description of the Related Art
Conventional methods used for display apparatuses such as a head-mounted display (HMD) and a head-up display (HUD) include a method for performing direct drawing on the retinas of the eyes of a user by performing two-dimensional scanning using a laser beam (hereinafter, described as a laser-scanning method) (for example, see Japanese Patent No. 2932636.) The display apparatus according to the laser scanning method is also known as a: retinal scanning display (RSD), retinal irradiation display, retinal direct-draw display, laser scanning display, direct-view-type display, virtual retinal display (VRD), and so on.
In general, in the HMD or HUD according to the laser scanning method, a scan unit performs two-dimensional scanning using a laser beam emitted from a laser light source, and a deflection unit, such as a lens or a mirror, provided in front of the eyes of the user deflects the beam toward the pupils. Then the beam passed through the pupils forms an image on the retinas.
Since the deflection unit is provided in front of the eyes of the user, there is a problem in that the user's breathing or sweating cause condensation on the surface of the deflection unit, resulting in fogging on the deflection unit. The fogging on the deflection unit decreases efficiency and precision of deflecting the beam projected by the scan unit toward the pupils of the user. As a result, a problem of image degradation occurs. Further, in the case where the deflection unit transmits visible light, such a problem occurs where transmittance is decreased and viewing external world becomes difficult.
Examples of common measures to remove fogging include a method which uses an infrared heater (for example, see Japanese Unexamined Utility Model Application Publication No. 4-65648) and a method in which a heat conductive layer is formed on the rear surface of a mirror (for example, see Japanese Patent No. 2908696). There is also a method which uses a transparent resin which absorbs infrared rays to reduce fogging (for example, see Japanese Unexamined Patent Application Publication No. 2000-143842).
Furthermore, examples of methods for detecting a line-of-sight includes a method in which infrared rays are projected onto the eyes and the reflected light is used for the line-of-sight detection (for example, see Japanese Patent No. 2995876), and a method in which a light, reflected from the eyes, of a laser beam projected for scanning, is used for the line-of-sight detection (for example, see Japanese Patent No. 3425818).
However, in the method using the infrared heater and the like as in Japanese Unexamined Utility Model Application Publication No. 4-65648, an infrared heater and the like is necessary in addition to the light source and the scan unit for displaying an image. Consequently, such problems occur that the total volume and weight of the HMD and HUD increase or component cost increases.